Method of shell molding



nited. assessmen This invention relates: to the founding art and more specifically to that portion of the founding art which is concerned with the casting of metals in the so-called shell molds. This of molding has been widely described in the recent technical literature- Basically, shell molding comprises the preparation of a metal receiving mold by showering a mixture of sand and thermosetting resin upon a'metal pattern heated to a temperature in the neighborhood of 350F450" F. -The sand is usually ordinary foundry sand of about #90 A. F. S.

size containing about I percent of partially cured phenolformaldehyde resin. The heat of the mold completes the cure of the thermo-setting resin in 'situ and causes the formation of a rigid "mold' which can be stripped from the heated pattern'and employed to receive molten metal.

While thisprocess "has proven very useful and has 4 come into widespread c'onimercialuse; certain inherent limitations have become apparent and 'this invention has resulted from efforts to circurnv'entsuch limitations.

To secure good contact between the heated pattern and the sand resin mixture it is the practice of the 'art to rely completely upon the'forcef'o'f gravity. To enable the completed moldto be] stripped from the pattern, it -has been found necessarytolubricate the pattern periodically with a heat stable lubricant, as for instance, a five percent silicone emulsion in water. These two factors have caused difficulties in building-up a proper shell thickness where the face of the pattern is approximately parallel to'the force of gravity. :Under' these circumstances, there is little or no 'force' tending to impel the sandresin mixture against the lubricatedvertical portions of the pattern. A similar problem exists when an effort is made to filla heated metal core box with the sand-resin mixture to cast a core. 'In the usual practice,"the heated core box is filled with the sand-resin'mixture-and after allowing sufiicient time-for the resin to'initially set, the core box'is inverted to reinovethe excess molding compound. This inevitably'resulted-in some portion of the newly formed hot plastic core being attached to an upper horizontal surface or' toaverticaf'surface and hence unsupported against gravity except 'byrits own rigidity and its adherence to the lubricated core boxrmThese two forces are often insufiicient to support the weight of the core and consequently a portion .of the core wall either deforms or sloughs off completelyv H: I It has been found that these difiiculties can be avoided if the mold or core-is formedin a two-step operation rather than a one-step operation; According to this inyentiom'andusingthecore box as an example, a core istformed .by filling the hot core ,box with'the sand-resin mixture,-quickly-dumping the excess mixture, permitting the thin shell so formed to curejf'r om the'heat'of'the core box and'then refilling the co're box with further sand-resin mixtureandl'pe'rmittingi this 'mi'xture to re-i main in place until abore'fof the desired thickness has been builtup. i

' When the initial "charge 'of sand-resin mixture is placed in a heated core box and then quickly dumped, there results a very thin shell of sand-resin mixture over the entire interior of the core box. Because this shell is very thin, the resin portion thereof quickly becomes heated and cures and results in a thin, light,.but comparatively strong shell. The inner surface of this shell is of course rough and forms an ideal surface to receive the final charge of sand-resin with perfect adherence. When the final charge of resin is placed into the shell lined core box, the heat from the core box penetrates the initial thin shell and cures the immediately adjacent resin to a depth dependent upon the time contact permitted. By regulating the length of time the second charge of sand-resin mixture is permitted in contact with the original shell, the thickness of the final shell can be regulated. When the desired thickness is attained, the core box is dumped, the resin permitted to cure further if necessary, the core box is split along the parting line and the completed core is removed.

Due to the inherent nature of this shell mold process, it is impossible to set definite numerical time and temperature limits for the two steps involved. As those skilled in the art will readily appreciate, the curing time of the resin is a direct function of the temperature to which it is exposed. Substantially the same results can be obtained by prolonged contact with a pattern or core box at a comparatively low temperature, or by brief exposure to such a metal surface at a rather elevated temperature. However, with a core box at a conventional temperature of 350 to '450" F., the first application of the sand-resin mixture is preferably limited to two secends and at these temperatures should, under no circumstances, exceed five seconds. Before this thin shell is burdened with a thicker shell necessary to retain molten metal, it should be permitted tocure for a period of time not exceeding twenty seconds. The final application of the sand-resin mixture may then be made and permitted to remain in place for a period of time normally about thirty seconds but which may be extended up to two minutes if necessary.

In addition to enabling the preparation of molds or cores otherwise impossible by shell molding, the use of laminated shells enables the conservation of the expensive thermo-setting resin. For example the resin content of the thin initial lamina of the mold can be increased from the usual seven percent to the range of ten to fifteen percent resulting in the production of a very thin but strong lamina. The resin content of the second and thicker lamina may then be lowered from seven percent to as low as three percent due to the ideal shell forming conditions obtaining after the formation of the initial lamina. By virtue of the much greater thickness of the second lamina, an overall economy of resin may be effected. Similarly under some circumstances it may be desirable to decrease the concentration of resin in the first sand-resin mixture and use the higher concentration of resin in the second lamina.

It is to be understood that the term mold as employed in the subjoined claims is to be construed to include structures conventionally referred to as cores.

What is claimed is:

l. The process of preparing a shell mold comprising applying to a metal surface heated to a temperature'of about 400 F; a loose mixture of sand and a thermosetting resin, permitting the loose sand-resin mixture to remain in contact with the heated metal surface for a period of time not exceeding two smonds to produce a thin self supporting lamina of sand-resin at least a portion of which is supported against gravity only by its own rigidity and adherence to the metal surface, promptly removing the excess loose sand-resin mixture, permitting the resin in the adhering lamina to cure further, reapplying more loose sand-resin mixture to substantially the entire area of the rough'surface of the thin lamina, main and curing the, laminatedmold so formed.

2, The process of preparing a shell moldcomprising applying to ametal surface heated to, av temperature of about, 400 F; a loose mixture of sand and a thermoseifing resin, permitting the loose sandnesin mixture to remain in contact with the, heated metal surface for a period of time not exceeding two seconds to produce a thinself-supporting lamina of sand-resin at least a, portion of which is supported against gravity only by its ownrigidityand adherence to the metalsurface, promptly removing the excess loose sand-resin mixture, permitting the resin in the adhering lamina to cure further, reapplying more loose sand-resin mixture to substantially the entire area of the roughsurface of the thin lamina, maintaining this loose sand-resin mixture in contact with the thin lamina until the desired thickness of shell has been produced, removing the excess loose sand-resin mixture and curing the laminated mold so formed, said first mentioned sandrresinmixture having a resin content of ten to fifteen percent and said second mentioned, sand-resin mixture having a resin contentfrom three to seven percent. t

3. The process of producing a laminated hollow shell core comprising filling, a heated core box with a loose mixture of sand and a thermosetting resin, permitting the resin portion of the sand-resin mixture adjacent the heated metal surface to cure, for a time not in excess of 5 seconds only sufficiently to produce a thin self-supporting lamina of sand-resin mixture at least a portion of which is supported against gravity only by its own rigidity and adherence to the wall of the core box, removing the excess of non-adhering loose sand-resin mixture, permitting the resin in the sand-resin lamina to cure further, again filling the heated core box with another loose sandresin mixture having a resin content substantially less than the first mentioned sand-resin mixture, maintaining this loose sand-resin mixture in contact with the thin lamina until the desired thickness of core has been produced,

removing the excess loose sand-resin mixture and curing metal surface to cure, for a time-not in excess of 5 secduds only sufliciently to produce a thin self-supporting lamina of sand-resin mixture at least a portion of which is supported against gravity only by its own rigidity and adherence to thewall of the core box, removing the excess of non-adhering loose sand-resin mixture, permitting the resin in the sand-resin lamina to cure further, again filling the heated core box with another loose sand-resin mixture having a resincontent substantially less than the first mentioned sand-resin mixture, maintaining this loose sand-resin mixture in contact with the thin lamina until the desired thickness of core has been produced, removingthe excess loose sand-resin mixture and curing the laminatedcore so formed, said first mentioned sand-resin mixture having a resin content of ten to fifteen percent and said second mentioned sand-resin mixture having a resin-content from three to seven percent.

5. The process of preparing a laminated hollow shell corecomprising-filling acore box heated to a temperature of about 400 F. with a loose mixture of sand and a thermosetting resin, permitting the loose sand-resin mixture to remain in contact with-the heated core box for a period of. time not exceeding two seconds to produce a thin self-supporting lamina of-sand-resin mixture at least a ;por tion ,-of; which,is supported against gravity only byits own rigidity, and adherence to the lv j im:

box, promptly removing the excess loose sand-resin mixture, permitting the resin in the adhering lamina to cure further, refilling the core box with a further mixture of sand and resin, maintaining this loose sand-resin mixture in' contact with the previously produced thin lamina until the desired thickness of shell hasbeen produced, removing the excess loosesand-resin mixture. and,curing the laminated moldso formed, said'first mentioned'sand resin mixture, having; at resincontent. of; ten, to-fifteen percent; and said'secondi mentioned sand-resin mixture having a resin contentof three to seven percent;

6. The processor producing alaminated hollow shell core comprising; filling; a. heated, core box with; a loose mixture of sand and aithermosettingresin, permitting the resin portion of the sand-resinmixture adjacent the heated core box to cure, for a time-not-in excess of 5 seconds only suificiently to produce a thin, self-supporting lamina of sand1re sin mixtureatleast a portion of. which-is. supported; against I gravityv only by its; own.; rigidityand. ad-- herenceto the walLof the core,box, removingthe.. excess. of nonradhering; loose;sand-resin mixture, permitting the resin inthe thinsand resin lamina to cure further, refilling the heated core boxwith more loose sand-resin mixture directly'in contactwiththe rough surface of the, pre

viously produced lamina, maintaining this, loose sand,

resinmixture in contact-with the thin previously formed.

lamina until the desired thickness-of shell has been proexceeding two seconds to produce a thin self-supporting lamina of sand-resinat least a-portion of which: is sup-- ported against gravity only by its own rigidity and adherence to the wall ofthe corebox, removing the; excess of non-adhering loose sandaresin mixture, promptly refilling the heated core boxwith more loosesand-resin mixtureso that it--is directly in contact with the rough surfaceof the previously formed lamina, maintaining this contact until thedesired, thickness of; shell has been produced, removing the excess loose sand-resin mixture and curingthe laminated core so formed.

8. In aprocessof producing a laminated hollow shell core from sand mixed; with a thermosetting resin which includes the steps of filling a heatedcore box with aloose mixture; of sand-and'thermosetting resin and thenpermit; ting substantially all of, the sand resin mixture to flow from the pattern to leave, athinlayer of partially cured sand resin mixture at least a portion of which issupported against gravity only by-its own rigidity and adherence to the wall of the core box, the improvement comprising permitting the resin portion of the sand resin mixture adjacent-the heated' metal surface to cure for a'time not in excess of five secondsand only sufficiently to produce a thin self-supporting lamina of sand resin. mixture, removing the excess of the'non-adhering loose sandresinmixture, permitting the resin in the sand resin-lamination so produced-to cure further, again filling the heated; corebox with a -loose sand resin mixture, maintaining this.

loose sand resin mixture in contact with the thin laminaa. tion until the=desired thickness of core has been produced,

removing the excess loose-sand resin mixture and'curing:

the-laminated'core so formed;

9. In a process;of -producing a-laminated hollow shell core from sand-'mixedwith a thermosetting resin which: includes thesteps of filling a heated core box with a loose the wall of the core box, the improvement comprising permitting the resin portion of the sand resin mixture adjacent the heated metal surface to cure for a time not in excess of five seconds and only sufiicicntly to produce a References Cited in the file of this patent UNITED STATES PATENTS 2,441,695 Feagin et al May 18, 1948 2,614,303 Duncan Oct. 21, 1952 2,772,458 Henry Dec. 4, 1956 FOREIGN PATENTS 832,934 Germany Mar. 3, 1952 OTHER REFERENCES Modern Metals, October 1950, pages 22-24. The Foundry, October 1950, pages 162, 164 and 168. 

8. IN A PROCESS OF PRODUCING A LAMINATED HOLLOW SHELL CORE FROM SAND MIXED WITH A T HERMOSETTING RESIN WHICH INCLUDES THE STEPS OF FILLING A HEATED CORE BOX WITH A LOOSE MIXTURE OF SAND AND THERMOSETTING RESIN AND THEN PERMITTING SUBSTANTIALLY ALL OF THE SAND RESIN MIXTURE TO FLOW FROM THE PATTERN TO LEAVE A THIN LAYER OF PARTIALLY CURED SAND RESIN MIXTURE AT LEAST A PORTION OF WHICH IS SUPPORTED AGAINST GRAVITY ONLY BY ITS OWN RIGIDITY AND ADHERRENCE TO THE WALL OF THE CORE BOX, THE IMPROVEMENT COMPRISING PERMITTING THE RESIN PORTION OF THE SAND RESIN MIXTURE ADJACENT THE HEATED METAL SURFACE TO CURE FOR A TIME NOT IN EXCESS OF FIVE SECONDS AND ONLY SUFFICIENTLY TO PRODUCE A THIN SELF-SUPPORTING LAMINA OF SAND RESIN MIXTURE, REMOVING THE EXCESS OF THE NON-ADHERING LOOSE SAND RESIN MIXITURE, PERMITTING THE RESIN IN THE SAND RESIN LAMINATION SO PRODUCED TO CURE FURTHER, AGAIN FILLING THE HEATED CORE BOX WITH A LOOSE SAND RESIN MIXTURE, MAINTAINING THIS LOOSE SAND RESIN MIXTURE IN CONTACT WITH THE THIN LAMINATION UNTIL THE DESIRED THICKNESS OF CORE HAS BEEN PRODUCED, REMOVING THE EXCESS LOOSE AND RESIN MIXTURE AND CURING THE LAMINATED CORE SO FORMED. 